Frequency domain characteristics of muscle sympathetic nerve activity in heart failure and healthy humans.

The purpose of this study was to characterize oscillations in muscle sympathetic nerve activity (MSNA) in the frequency domain in healthy subjects and patients with congestive heart failure (CHF) and to relate these to blood pressure (BP), heart rate (HR), and breathing frequency. MSNA burst frequency was significantly greater in CHF [52 +/- 21 (n = 12) vs. 35 +/- 11 (n = 19) bursts/min, P < 0.05], whereas breathing frequency and HR were similar. There was no significant difference between CHF and healthy subjects in total power, harmonic power, and nonharmonic power in the MSNA spectrum from 0 to 0.5 Hz, but low frequency power (LF, 0.05-0.15 Hz, P < 0.05) was reduced in heart failure patients. There was less coherence between BP and MSNA in the LF range, but similar spectral power in both groups in the very LF (VLF, 0-0.05 Hz) and high frequency (0.15-0.5 Hz) ranges. The transfer of MSNA oscillations into BP in the VLF (P < 0.05) and LF (P < 0.02) ranges was significantly lower in CHF, but gains in the transfer function and in the coherence between BP and MSNA and in the coherence between respiration and MSNA were similar in the two groups. These observations indicate that modulation of MSNA by the arterial baroreflex and respiration is preserved in CHF. The loss of LF power in the MSNA signal may be due to impaired neuroeffector transduction. The higher sympathetic nerve firing rate in CHF would therefore appear to be caused by factors other than the loss of regulation by these two inhibitory influences.